Zr alloys are in widespread use in the fuel assemblies of nuclear reactors for making parts that are subjected to severe conditions of irradiation, of mechanical stress, and of corrosion. Such parts include cladding containing fuel pellets, boxes, grids, and various spacer elements, for example.
Various categories of Zr alloy have been developed, corresponding to various requirements of users depending on the looked-for properties of the various parts. These depend on the mechanical, thermal, and physicochemical (irradiation, corrosion) stresses to which they are subjected while in use in a reactor.
Amongst these alloys, some have the feature of containing significant quantities of Nb. A description can be found in particular in U.S. Pat. No. 4,649,023, where they are applied to fabricating tubes for light water reactors, both for boiling water reactors (BWR) and for pressurized water reactors (PWR).
Other documents (U.S. Pat. No. 5,266,131) envisage applying them to other parts that are made from sheets. However, until now, industrial applications for such alloys containing Nb have been limited to pressurized water reactors (PWR). Attempts at using the same alloys in boiling water reactors (BWR) have not yet been conclusive, since their behavior in terms of general corrosion and in terms of nodular corrosion is not satisfactory. It is thus common practice in BWRs to use other types of Zr alloy.
Proposals are made in document JP-A-62 182 258 for fuel assembly components, in particular for BWRs, to be made of a Zr—Nb—Sn—Fe—O alloy obtained by cold rolling, followed by β (or α+β) quenching, followed by work hardening by at least 30%, and then by aging at a temperature greater than the recrystallization temperature (e.g. 450° C.-550° C.), without subsequent cold rolling. That produces a structure having fine precipitates of βNb and of ZrFe2 intermetallic compounds. The idea is thus to obtain parts that are relatively insensitive to nodular corrosion and that have high toughness and ductility.
Proposals have recently been made (document WO-A-2006/004499) to use, in a BWR, alloys that contain Nb for making components from sheet metal. No alloy element is to be present therein at an amount greater than 1.6%. The thermomechanical treatments performed on the alloy lead to substantially all of the secondary phase particles being transformed into particles of βNb containing at least 90% Nb. Preferably, the Fe content of the alloy lies in the range 0.3% to 0.6% by weight, and apart from Zr, Nb, and Fe, the alloy contains only Sn in significant amount. The content of any other alloy element must not exceed 500 parts per million (ppm). Those alloys seek to provide good resistance to conventional types of corrosion and to irradiation growth.
Nevertheless, a problem that is frequently encountered in BWRs is associated with the appearance of so-called “shadow corrosion”.
That is a type of corrosion that arises when two parts made of materials of different kinds are coupled galvanically (electrons being transferred between the two materials that are immersed in a medium presenting non-zero electrical conductivity) in the presence of oxidizing species. Specifically, the conductive medium is the boiling water of the reactor. When the coupling occurs between a Zr alloy component (such as a box or fuel cladding) and a component made of an alloy based on Ni or of stainless steel (such as a grid for spacing the tubes apart), then localized white corrosion is observed to appear on the Zr alloy, on surfaces that correspond to shadows of other parts made of Ni based alloy or stainless steel. The phenomenon is amplified by irradiation that modifies the physicochemical characteristics of materials and creates oxidizing species on the surfaces of components by radiolyzing the heat-conveying fluid, in addition to the species created by the oxygen dissolved in the boiling water of the reactor. The quantity of dissolved oxygen is much greater than that present in the pressurized water of PWR reactors. BWR fuel assemblies are very sensitive to this type of corrosion, and the solutions that have been developed in the past for reducing or eliminating such localized corrosion consists, for example, in coating one of the components present so as to make it electrochemically compatible with the other (see document US-A-2006/0045232).